The 3‐hour K index was the first to provide an objective and quantitative monitoring of the irregular variations of the transient geomagnetic field observed in a given place. The use of K indices from a network of observatories to derive a planetary index of geomagnetic activity was suggested by Bartels when defining these indices. Then, Kp, am, Km, an, as, and aa were successively designed and accepted as International Association of Geomagnetism and Aeronomy indices. At present these K‐derived planetary indices are routinely computed and circulated. They make long homogeneous data sets and are widely used for long‐term and statistical studies in geomagnetism and solar‐terrestrial physics. After a short description of the main features of transient geomagnetic activity a definition of the K index as a measure of the irregular activity is given with a summary of its basic characteristics. The derivation of K‐derived planetary indices is described and discussed, and updated indications concerning their availability are presented. This short review provides users with the minimum required knowledge about these indices and may serve as an introduction to the Mayaud (1980) monograph.
The annual variation of magnetic activity and its variation with universal time are analyzed separately for days sorted according to the polarity of the interplanetary magnetic field (IMF). A coherent interpretation of the observed effects is obtained (1) by taking into account the variations of the southward component of IMF in GSM (geocentric solar magnetospheric) coordinates, as was suggested by Russell and McPherron (1973) (the effect expected from this mechanism is calculated and compared to the observed ones), and (2) in the case of Am, by taking into account the annual and diurnal variations of the angle between the dipole axis and the earth‐sun line.
This paper is devoted to a reevaluation of the annual and universal time variations of the geomagnetic activity through a study of the am index over a 30‐year period starting in 1959, taking into account the interplanetary magnetic field polarity. A visualization of the annual diurnal variations, showing the continuous evolution of the times of the diurnal maximum and minimum, is applied both to this extended data set and to the simulation of the main processes invoked to explain these variations. The various contributions of the different processes to the annual‐diurnal variations of the activity are discussed, including the Russell‐McPherron and the McIntosh effects, as well as a third source introduced in this study. One concludes that three contributions, two of which being attached to the Bz southward influence, must be combined to account for the observations.
In this paper, we study the relationship between the spatial variations of the perpendicular electric field observed in the topside ionosphere and those of the magnetic field induced by the associated field-aligned currents. The mapping of the magnetospheric electric field down to the ionosphere depends upon the spatial scale of its variations and, consequently, also the amplitude of the perpendicular currents and their divergence. This mapping is modeled using realistic conductivity profiles, and the dependence of the electric and magnetic fields upon the spatial scale is analyzed. We compare the effective integrated Pedersen conductivity •P,eff defined in our model to zlB //.t o AE, where zlE and zlB are the correlated variations of the mutually orthogonal electric and magnetic fields in the topside ionosphere. We have studied the variation of •P,eff as a function of the scale of zlE and zlB fluctuations. •P,eff is constant and equal to the chssical integrated Pedersen conductivity for scales larger than = 5 km. At smaller scales, it shows a steep decrease down to the lowest scales considered of 0.1 km. The good agreement between observations made on board the AUREOL 3 satellite and this model calculation suggests that the ULF electromagnetic turbulence observed frequently in the cusp and auroral zone may be essentially due to the crossing of spatially structured field-aligned currents. The limits of the present model and its possible improvements are discussed. INTRODUCIION The ionospheric closure of field-aligned currents and the mapping of the perpendicular electric field from high altitudes down to the lower ionosphere and dynamo region have already been studied by a number of authors. For example, Swift [1972], considering the case of a vertical magnetic field typical of high-latitude regions, has shown that the usual integrated Pedersen conductivity, or Pedersen conductance = I: a(z) dzmay be used to relate the horizontal Pedersen current to the horizontal electric field at high altitudes for spatial wavelengths larger than = 10 km (hm is the base of the dynamo region). The case of smaller wavelengths has been examined only to describe the mapping of the electric field, either between the ionosphere and the magnetosphere [Farley, 1959[Farley, , 1960Spreiter and Briggs, 1961;Reid, 1965;Zhang and Cole, 1987] or between the ionosphere and the lower atmosphere [Chiu, 1974]. These authors have shown that the mapping factor, equal to the ratio of the ionospheric to magnetospheric electric fields, or of the atmospheric to ionospheric electric fields, varies with the transverse scale of the variations of the electric field, and that the coupling between the ionosphere and the magnetosphere does not persist for wavelengths smaller than = 1 km. The integrated conductivity must then be replaced by an "effective" integrated conductivity, which characterizes the region of the ionosphere actually affected by the mapping. This concept was originally introduced by Scholer [1970] in a study of the deceleration of artifici...
Results obtained by the Aureol 3 satellite on electromagnetic waves in the ELF range in the high‐latitude topside ionosphere show the existence of a new type of turbulence. This turbulence is observed on both the electric and magnetic wave components and appears as bursts of 0.1 s to a few seconds typical duration in the polar cusp and auroral zone, associated with field‐aligned currents and low energy electron precipitations. The two possible interpretations of these observations, either in terms of a filamentary structure of the field‐aligned currents or of Alfven waves are discussed. For either of these models the observed values of the parallel component of the electric field are much larger than those predicted by classical models of the electrodynamics of the lower ionosphere. These results are compared with recent observations and might indicate the existence of acceleration mechanisms at low altitudes.
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